to reproduce 

1. On iPhone 13mini or equivalent disable WiFi
2. Open Minecraft app, login to your Microsoft account
3. Go to servers tab and try to connect to any server

what happens

a pop-up message appears:
It appears you are unable to connect to your network, please check your connection.

the app won’t connect to any server

what should happen

the app connects to a server

additional info

The marketplace works fine, connect to servers over WiFi works fine.
tested on O2 cellular network (UK, GiffGaff)